1. Field of the Invention
The present invention relates to a magnetic memory.
2. Related Background Art
An MRAM (magnetic Random Access Memory) has gained attention as a nonvolatile memory. The MRAM is a storage element which stores information by controlling the direction of magnetization of the free layer (magnetosensitive layer), and reads the stored information by measuring the quantity of electrons transmitted through the free layer. Initially, in writing on a storage element, an electric wire is arranged near the storage element, and the direction of magnetization of the free layer is changed by an inductive magnetic field generated by a current supplied into the electric wire. However, writing using an inductive magnetic field easily causes erroneous writing on an adjacent storage element. In this method, magnetic energy diverges into spaces, so that the writing efficiency is low and the writing current is large.
Therefore, a magnetic path is constructed by using a soft magnetic material or the like, and a magnetic field is supplied in a concentrated manner to the storage element through the inside of the magnetic path, whereby the writing efficiency can be improved. However, to drive the soft magnetic material, large energy is necessary, so that it is difficult to greatly increase the writing efficiency.
Therefore, recently, a writing method using spin injection has attracted attention. That is, the direction of magnetization of the free layer can be changed by the spin injection. Magnetization means a state that the direction of electrons spinning are the same in the magnetic material. When electrons spinning in reverse are injected into the inside of the magnetic material, the direction of magnetization inside the magnetic material reverses (spin injection magnetization reversal) according to the injected spin. To cause the spin injection magnetization reversal, it had been generally considered that a comparatively large current is necessary, however, it has been known that magnetization reversal is caused even by a comparatively small current.
As scientific studies in this field, studies described in Non-patent Document 1, Non-patent Document 2, and Non-patent Document 3 are known. In Non-patent Document 1, a device having a plurality of magnetic material layers is described, and torque in the case of a mirror-symmetric structure is discussed. Non-patent Document 2 discloses a spin transistor, however, magnetization reversal in this transistor is performed by using an external magnetic field. Non-patent Document 3 describes a spin torque in a system including two magnetic films laminated via a nonmagnetic film.
Next, a writing method using spin injection will be described.
FIG. 11 is a sectional view of a conventional magnetoresistance effect element (memory element) using spin injection.
The memory element 10 includes a free layer 3, a pinned layer 1 made of a ferromagnetic material, and an intermediate nonmagnetic layer 2 interposed between the free layer 3 and the pinned layer 1. On the surface of the free layer 3 opposite to the intermediate nonmagnetic layer 2, a spin filter formed of a nonmagnetic layer 4 and a ferromagnetic layer 5 is provided.
When writing data, a current flows in a thickness direction of the layers by supplying a bias voltage between the terminal A and the terminal B. By applying a current, spin having a specific polarity of magnetization injected into the inside of the free layer 3 via a spin filter or by being reflected by the spin filter torques the direction of magnetization of the free layer 3, and this direction of magnetization coincides with the polarity of the spin.
At the time of data reading, when the direction of magnetization of the pinned layer 1 and the direction of magnetization of the free layer 3 are parallel to each other, the spin polarized current passing through the intermediate nonmagnetic layer 2 is large, and in this case, for example, “1” is written. When the direction of magnetization of the pinned layer 1 and the direction of magnetization of the free layer 3 are anti-parallel to each other, the spin polarized current that passes through the intermediate nonmagnetic layer 2 is small, and in this case, for example, “0” is written.
Herein, the direction of magnetization of the pinned layer 1 and the direction of magnetization of the ferromagnetic layer 5 are anti-parallel to each other.
FIG. 12 is a circuit diagram of a magnetic memory formed by aligning a plurality of memory elements 10 described above.
When an H level control signal is applied to the word line W1, the transistor Q1 is turned ON, and a current flows from the bit line B1 to the ground via the memory element 10 and the transistor Q1. The potential of the bit line B1 is controlled by the X-coordinate designating circuit 20, and the potential of the word line W1 is controlled by the Y-coordinate designating circuit 30.
At the time of data reading, when the potentials of the word line W1 and the bit line B1 of a specific address are both raised by these circuits, the transistor Q1 is turned ON, and a current corresponding to a resistance value of the memory element 10 positioned at this address flows into the bit line B1. This current is applied to the resistor, and a voltage drop of this resistor is inputted into a comparator, whereby from the comparator, a digital value corresponding to the magnitude of the current, that is, information stored in the memory element 10 is outputted.
At the time of digital value writing, when the potential of the word line W1 of a specific address is raised by the above-described circuit, the transistor Q1 is turned ON, and at this point in time, when the potential of the bit line B1 is greatly raised or lowered, a current flows into the memory element 10 positioned at this address. This current is set to be larger than that for reading. Therefore, spin injection is performed into the inside of the memory element 10, and according to the injected spin polarity, the direction of magnetization of the free layer is determined.
FIG. 13 is a graph showing a relationship between the current I to be supplied to the memory element 10 and the MR ratio (resistance change rate).
In this graph, a hysterisis curve is drawn. As seen in the graph, when a current whose absolute value is not less than a threshold ITH is supplied, the direction of magnetization of the free layer can be reversed. That is, for data writing, a current with a magnitude (not less than ΔIR and not more than ΔIW) which causes spin injection magnetization reversal is supplied to the memory element 10, and for data reading, a current with a magnitude (less than ΔIR) which does not cause spin injection magnetization reversal is supplied to the memory element 10.
Non-patent Document 1: L. Berger, “Spin-wave emitting diodes and spin diffusion in magnetic multilayers,” IEEE Trans. Mag. Vol. 34, Issue 6, pp. 3837-3841, 1998
Non-patent Document 2: M. Johnson, “Bipolar spin switch,” Science, Vol. 260, pp. 320-322, 1993
Non-patent Document 3: J. C. Slonczewski, “Current-Driven Excitation of Magnetic Multilayers,” Journal of Magnetism and Magnetic Materials, Vol. 159, L1-L7, 1996